Contrary to general belief, spermatogenesis in wild white-footed mice and deer mice (Peromyscus sp.) at the latitude of northern Penna. begins in late November or early December and is preceded by gonadal regression to prepuberal condition in late October and early November. However, seminal vesicles do not increase in weight until February, suggesting an appreciable time lag between the onset of spermatogenesis and the first evidence of testosterone secretion. If measurement of gonadotrophins and plasma testosterone levels, augmented by in vitro incubation of the testes, indicates that the time lag is real, we may have a way of learning more about the hormonal control of spermatogenesis. We also plan to manipulate photoperiod in the laboratory in order to understand better the relationship between spermatogenesis and photoperiod. Animals will be kept on normal photoperiod, reversed annual cycle of daily photoperiod, in no light and in constant light. Pinealectomized mice will be kept on normal photoperiod to determine if photoperiod acts through the pineal to cause gonadal regression and/or onset of spermatogenesis in short days. Also mice will be kept in pens outdoors and sampled regularly and we will periodically collect mice of known, as well as unknown age, for hormonal studies. Spermatogenesis will be evaluated histologically using the techniques of Clermont and Leblond.